Image sensors are used in cameras and other imaging devices to capture images. In a typical imaging device, light enters at one end of the imaging device and is directed to an image sensor by an optical element such as a lens. In most imaging devices, one or more layers of optical elements are placed before and after the aperture stop to focus light onto the image sensor. Recently array cameras having many imagers and lenses have been developed. In most cases, multiple copies of the optical elements must be formed laterally for use in array cameras.
Conventionally, optical arrays can be formed by molding or embossing from a master lens array, or fabricated by standard lithographic or other means. However, the standard polymer-on-glass WLO and monolithic lens WLO manufacturing techniques have so far not been adapted for the specific high performance requirements of array cameras. In particular, some technical limitations of conventional WLO-processes need to be reduced, such as, for example, minimum substrate thickness requirements, inflexibility of where to place the aperture stop, accuracy, etc. The flexibility of such choices or processes needs to be increased in order to meet the high demands by array cameras otherwise such WLO techniques cannot be used to manufacture array cameras. Accordingly, a need exists for fabrication processes capable of accurately forming these arrays and for optical arrangements that give an increased flexibility in manufacturing so that the image processing software of these new types of array-type cameras can take advantage to deliver superior image quality at the system level.
The optical transfer function (OTF) of an imaging system (camera, video system, microscope etc.) is considered the true measure of an imaging system's performance, i.e., the resolution (minimum feature size or maximum spatial frequency that can be imaged with sufficient contrast) or image sharpness (the contrast at a given spatial frequency) obtainable by an imaging system. While optical resolution, as commonly used with reference to camera systems, describes only the number of pixels in an image, and hence the potential to show fine detail, the transfer function describes the ability of adjacent pixels to change from black to white in response to patterns of varying spatial frequency, and hence the actual capability to show fine detail, whether with full or reduced contrast. The optical transfer Function (OTF) consists of two components: the modular transfer function (MTF), which is the magnitude of the OTF, and the phase transfer function (PTF), which is the phase component.
In cameras, the MTF is the most relevant measurement of performance, and is generally taken as an objective measurement of the ability of an optical system to transfer various levels of detail from an object to an image. The MTF is measured in terms of contrast (degrees of gray), or of modulation, produced from a perfect source of that detail level (thus it is the ratio of contrast between the object and the image). The amount of detail in an image is given by the resolution of the optical system, and is customarily specified in line pairs per millimeter (Ip/mm). A line pair is one cycle of a light bar and dark bar of equal width and has a contrast of unity. Contrast is defined as the ratio of the difference in maximum intensity (Imax) and minimum intensity (Imin) over the sum of Imax and Imin, where Imax is the maximum intensity produced by an image (white) and Imin is the minimum intensity (black). The MTF then is the plot of contrast, measured in percent, against spatial frequency measured in Ip/rm. This graph is customarily normalized to a value of 1 at zero spatial frequency (all white or black).